File:Origin of Vertebrates Fig 167.png

English: Fig. 167.—A, Diagram of Section through a Trilobite-like Animal; B, Diagram to illustrate the Suggested Formation of the Notochord from a Ventral Groove; C, Diagram to illustrate the Suggested Formation of the Post-Branchial Gut by the continuation of the same process of Ventral Groove-Formation, combined with Obliteration of Appendages and Growth of Pleural Folds; D, Diagram to illustrate the Completion of the Vertebrate Type by the Meeting of the Pleural Folds in the Mid-Ventral Line with the Obliteration of the Atrial Cavity and the Conversion of the Ventral Groove into the closed Alimentary Canal.

Al., alimentary canal; N., nervous system; My., myotome; Pl., pleuron; App., appendage; Neph., nephrocœle]; Met., metacœle; Sd., segmental duct; Mes., mesonephros; At., atrial chamber; Nc., notochord; H., heart; F., fat body; Ng., notochordal groove. (These diagrams are intended to complete the diagrams in Fig. 160, which were purposely left incomplete.)